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Fu-Kwun Hwang
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 « Embed this message on: February 25, 2011, 10:53:27 pm » posted from:Taipei,T\'ai-pei,Taiwan

The circuit below shows a generalized form of a differential amplifier with two inputs marked V1 and V2. The two identical transistors TR1 and TR2 are both biased at the same operating point with their emitters connected together and returned to the common rail, -Vee by way of resistor Re.
The circuit operates from a dual supply +Vcc and -Vee which ensures a constant supply. The voltage that appears at the output, Vout of the amplifier is the difference between the two input signals as the two base inputs are in anti-phase with each other. So as the forward bias of transistor, TR1 is increased, the forward bias of transistor TR2 is reduced and vice versa. Then if the two transistors are perfectly matched, the current flowing through the common emitter resistor, Re will remain constant.

Like the input signal, the output signal is also balanced and since the collector voltages either swing in opposite directions (anti-phase) or in the same direction (in-phase) the output voltage signal.

Assume the gain is G, the AC voltage variation for Rc at the left side is
$V_{1o}=-G*V_1$

The same for right side is $V_{2o}=-G*V_2$

The output is the difference between the above two signal.
$V_{out}=V_{1o}-V_{2o}=-G*(V_1-$ $V_2)$

The following simulation is designed to help you play with the differential amplifier. Enjoy it!

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